Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

[Will Deacon]

On Tue, Jun 24, 2014 at 06:57:44PM +0100, Olav Haugan wrote:
> On 6/24/2014 2:18 AM, Will Deacon wrote:
> > On Sat, Jun 21, 2014 at 12:16:25AM +0100, Olav Haugan wrote:
> >> We have multiple-master SMMUs and each master emits a variable number of
> >> StreamIDs. However, we have to apply a mask (the ARM SMMU spec allows
> >> for this) to the StreamIDs due to limited number of StreamID 2 Context
> >> Bank entries in the SMMU. If my understanding is correct we would
> >> represent this in the DT like this:
> >>
> >> 	iommu {
> >> 		#address-cells = <2>;
> >> 		#size-cells = <0>;
> >> 	};
> >>
> >> 	master@a {
> >> 		...
> >> 		iommus = <&iommu StreamID0 MASK0>,
> >> 			 <&iommu StreamID1 MASK1>,
> >> 			 <&iommu StreamID2 MASK2>;
> >> 	};
> > 
> > Stupid question, but why not simply describe the masked IDs? What use does
> > the `raw' ID have to Linux?
> 
> We do describe the masked StreamID (SID) but we need to specify the mask
> that the SMMU should apply to the incoming SIDs, right?
> 
> We have a bus master that emits 43 unique SIDs. However, we have only 40
> SMMU_SMRn registers in the SMMU. So we need to mask out some of the
> incoming SID bits so that the 43 SIDs can match one of 40 entries in the
> SMR.

Hmm, so you're talking about stream matching, right? That doesn't belong in
the device-tree. I appreciate that the current driver does a terrible job at
allocating the SMRs (it's bloody difficult!), but we should try to improve
the dynamic behaviour instead of moving configuration of the SMMU out into
device-tree, where it's inflexible at best.

There have been patches previously posted by Andreas Herrmann helping here.
I'd be glad to see them revived.

Will
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